Environmentally Assisted Cracking: Theory and Practice: Stress Corrosion Cracking II
Sponsored by: TMS Structural Materials Division, TMS: Corrosion and Environmental Effects Committee
Program Organizers: Bai Cui, University of Nebraska Lincoln; Raul Rebak, GE Global Research; Srujan Rokkam, Advanced Cooling Technologies, Inc.; Jenifer Locke, Ohio State University

Tuesday 2:30 PM
March 1, 2022
Room: 201D
Location: Anaheim Convention Center

Session Chair: Haozheng Qu, GE Global Research

2:30 PM
A New Test Specimen to Determine Environmentally-assisted Cracking Threshold: James Sobotka¹; Carl Popelar¹; Fassett Hickey¹; Julian Hallai²; Yifei Zeng³; ¹Southwest Research Institute; ²Exponent; ³ExxonMobil
    This presentation describes the development of a new test specimen that supports the determination of environmentally-assisted cracking threshold. This novel specimen has been designed to provide near-constant stress-intensity factor values under a constant displacement imposed by wedge loading. Consequently, this test specimen does not require active monitoring over long periods in hazardous sour environments. This specimen allows the threshold stress-intensity factor to be more readily based on crack initiation. This approach eliminates ambiguities associated with measurements of the arrest toughness. This work focuses on applications for sulfide stress cracking in tough, strong steels used for oil and gas applications. In this work, we discuss our design methodology, constraints on the specimen, failed specimen geometries, and the final specimen that permits the rapid development of crack threshold databases.

2:50 PM
Predicting Electrochemical Conditions in a Stress Corrosion Crack Tip and the Influence of Exposure Environment: Ryan Katona¹; Erin Karasz¹; James Burns²; Charles Bryan¹; Rebecca Schaller¹; Robert Kelly²; ¹Sandia National Laboratories; ²University of Virginia
     Metallic structures are commonly exposed to marine atmospheric conditions, characterized by thin water layers (WL), allowing for corrosion to occur. For austenitic stainless steels, pitting and stress corrosion cracking (SCC) are potential degradation mechanisms. The rate and extent of corrosion and SCC on the alloy surface is dictated by the combination of environmental, physicochemical, and geometric variables. Due to the large potential variation in environmental conditions, modeling can serve as an important tool to evaluate environmental effects. Finite Element Modeling is used to evaluate electrochemical conditions in a crack exposed to marine environments. A reactive transport model was developed to evaluate SCC crack tip conditions. The influence of WL thickness, chloride concentration, stress intensity, crack length, and specimen geometry are evaluated. The modeling results will give insight into testing phenomena.SNL is managed and operated by NTESS under DOE NNSA contract DE-NA0003525. SAND2021-8237 A.

3:10 PM
Environmental Fracture of Alpha-brass in a Multi-scale QM/MM Approach: Antoine Clement¹; Thierry Auger¹; ¹CNRS
    The environmental fracture of solid solution alpha-brass in the 0-40% zinc concentration range is of generic interest. An EAM (Embedded Atom Method) potential was developed for these alloys (cfc configuration) to simulate fracture at the atomic scale within the QM/MM (Quantum Mechanics/Molecular Mechanics) framework. It was fit to physical quantities calculated within a DFT-GGA approximation (Elastic constants, lattice parameter, cohesive energy among others for copper, zinc and the D023 Cu3Zn ordered alloy). This strategy ensures good continuity at the boundary within the QM/MM approach. The bulk physical properties predictions of this new binary alpha-brass alloy potential will be presented targeting mechanical behaviour (elastic anisotropy, phonon spectrum, stacking fault, solid solution hardening, ...). Preliminary results of a QM/MM modeling (using EAM/DFT-GGA) of the effect of the chemistry at the crack tip on the brittle or ductile response will be presented as a modelling framework for liquid metal embrittlement of alpha-brass alloys.

3:30 PM
Optimization of Stress Corrosion Mitigation in Al-Mg via Zn-Rich Primers: Matthew McMahon¹; Eric Dau¹; Allison Akman¹; ¹Naval Surface Warfare Center, Carderock Division
    The Al-Mg alloy AA5456 achieves a unique combination of weldability, general marine corrosion resistance, and high strength-to-weight ratio when utilized at relatively low temperatures in service. However, prolonged exposure to temperatures of at least 40ᵒC promotes Mg diffusion to the grain boundaries to form the highly anodic β phase in a process termed sensitization, which promotes stress corrosion cracking (SCC) and premature failure in seawater conditions. When these conditions cannot be avoided, a versatile cathodic protection system is useful to reduce or eliminate the rapid β dissolution and potential for SCC. Previous work demonstrated that inorganic Zn-rich primers have a unique capability to fill this role. This work will evaluate the efficacy of these primers in full immersion conditions in order to understand if specific inorganic Zn-rich primer attributes exist that enable their uniquely high level of cathodic protection performance and SCC mitigation on highly sensitized AA5456.

3:50 PM
The Effect of Loading Rate on the Environment-assisted Cracking Behavior of Sensitized AA5456-H116: Zachary Harris¹; James Burns¹; ¹University of Virginia
    The influence of the applied loading rate (dK/dt) on the environment-assisted cracking (EAC) behavior of sensitized AA5456-H116 in 0.6 M NaCl at applied potentials ranging from -800 to -900 mV_SCE is assessed via a rising-K testing framework. Results demonstrate that the effect of dK/dt is dependent on the applied potential, with minimal influence noted for potentials more positive than -830 mV_SCE, while a stronger dK/dt-dependence is observed for potentials more negative than -830 mV_SCE. This applied potential dependence is found to be consistent with the established mechanism for EAC in sensitized 5xxx-series aluminum alloys exposed to marine environments. Crack growth rates measured using rising versus static K testing are then compared, demonstrating that rising K methods consistently yield conservative EAC metrics with increased efficiency. The implications of these results are then discussed in the context of current standardized approaches for quantifying EAC susceptibility.